Distinct aggregates of small particles, or assemblies, characterize the plasma membranes of astrocytes. Having established the baseline of the number, shape and area of assemblies in developing and adult rat astrocytes, plasma membranes of reactive astrocytes are being examined for any morphological changes. The most striking difference in the cell membrane of reactive astrocytes is a change in the shape and an increase in the size of assemblies, especially within the plasma membranes of excrescences jutting into the subarachnoid space from the most superficial, marginal astrocytes. We are also examining assembly properties by adding various chemicals and drugs to primary cultures of astrocytes from 5-7 day old rats. In such astrocytes exposed to the protein inhibitor cycloheximide, assemblies are lost from the plasma membranes by 3 hours, although background particles and gap junctions persist. This evidence implies that assemblies are a protein with a high turnover rate. The immunocytochemical localization of neuron-specific enolase (NSE) has enabled us to distinguish, in vitro, small neurons from glial cells, which contain non-neuronal enolase (NNE). During development, migrating neurons either contain NNE only, or no detectable enolases. Only when the neurons become differentiated do they acquire NSE.